The present invention relates to a propulsion system for an airship or hybrid aircraft. More specifically, the present invention relates to a propulsion system which is capable of functioning as both a maneuver thruster and as an emergency ballonet fan for an airship or hybrid aircraft.
An airship is a buoyant vehicle, which can be propelled and steered through the air. Airships stay aloft primarily by means of a large cavity, enclosed by a balloon or envelope, which is filled with a lighter-than-air gas, such as helium. Since the airship is lighter than the air it displaces, it floats. A hybrid aircraft also includes an envelope and attains partial buoyancy from a lighter-than-air gas. However, a hybrid aircraft is heavier than air and is shaped like a wing. In addition to the lighter-than-air gas, a hybrid aircraft gets lift from aerodynamic flow over the envelope. Therefore, a hybrid aircraft has attributes of both an airship and an aircraft, such as an airplane.
The envelope of both an airship and a hybrid aircraft includes a number of air sacs or ballonets, which are used to control the buoyancy of the vehicle and maintain the shape of the envelope in response to expansion of the lighter-than-air gas due to changes in altitude or pressure. The ballonets are periodically filled with “heavy” air by ballonet fans. To begin the airship or hybrid aircraft's ascent, air is vented from the ballonets increasing the vehicle's buoyancy. As the vehicle rises, the ambient air pressure decreases, which causes the contained lighter-than-air gas to expand and maintain the size and shape of the envelope.
When the airship or hybrid aircraft is airborne, a propulsion system is used to control vehicle attitude and provide thrust. The propulsion system is comprised of a number of maneuver thrusters mounted outside of the envelope. Each maneuver thruster includes a propeller, which is powered by a motor, and a cowl, which encases and protects the propeller. In order to balance the pressure inside the envelope while in flight, air may be periodically pumped into and vented out of the ballonets to keep the vehicle neutrally buoyant in response to pressure and altitude changes. To descend, the ballonets are filled with air via the ballonet fans to increase the density of the vehicle. During descent, the ambient air pressure once again increases and additional air may be blown into the ballonets, thus providing the requisite pressure within the envelope.
As the airship or hybrid aircraft increases or decreases in altitude, it is important to maintain balance between the ambient air pressure and the pressure of the lighter-than-air gas inside the envelope. If the appropriate air pressure within the envelope is not maintained, a catastrophe can result. For instance, if the vehicle experiences some type of mechanical failure and begins to descend rapidly, the ambient air pressure will increase too quickly causing the envelope to collapse, which may result in structural damage. If this occurs, the airship or hybrid aircraft will essentially fall from the sky.
Various airworthiness authorities have specific requirements which address this type of emergency descent situation. In order to maintain the structural integrity of the envelope in this rare situation, these authorities mandate that every airship or hybrid aircraft has the ability to pump air into the ballonets at a high flow rate. In an effort to meet this requirement, current designs for airships or hybrid aircrafts include a number of emergency ballonet fans (in addition to the ballonet fans that operate under normal conditions), which are capable of pumping air into the ballonets very quickly. Thus, the emergency ballonet fans serve the sole purpose of rapidly inflating the ballonets in the event of an emergency descent situation and are several times larger and heavier than the ballonet fans needed for normal operation. The need to have these additional ballonet fans for use only in an emergency is costly and also results in an increase in the total weight of the vehicle.
Therefore, it would be useful in the art to provide a propulsion system that controls vehicle attitude and provides forward thrust under normal conditions and is capable of performing the function of emergency ballonet fans in an emergency descent situation.